Vine weevil larvae_m

During the re-potting season, it is an ideal opportunity to check the health of your bonsai tree roots and soil conditions. At the same time be on the look out for these little blighters who can have a fairly deviating impact on your tree roots, if they are allowed. If you find these little white grubs it is highly likely they are the larvae of the Vine Weevil, Otiorhynchus sulcatus. These will need to be removed from the bonsai soil.

Larave stage
Vine weevil larvae are white legless and plump grubs with a brown head up to 10mm in length with a slight curved body.
They feed on roots within the soil and also on corns and tubers and they particularly like potted plants as this creates a warmer environment.
The impact of the root predation is that the plant starts to show signs of lacking vigor as the root plate is affected making it less functional to the plant or tree. The plant can also start to wilt due to the root severance and damage and in severe cases there are insufficient roots to sustain the plant or tree and the result is death.
The larval damage is usually found between early autumn through to spring until the larvae develop into the beetle stage.
Control of the larval stage
There are a range of insecticides readily available from garden centers but repeat applications may be required to achieve the end result to kill off the larvae stages within the soil. Examples include Provado and Scotts Bug Clear Ultra Vine Weevil killer, which contain Thiacloprid and come in the form of a liquid drench.
The liquid drench is the most effective form of control for the soil borne larvae and should be used in mid to late summer as a preventative control. However be careful using chemicals and ensure you adjust the dosage for the use on bonsai trees and avoid using on flowering trees due to the impact on pollinating insects like bees.
The most effective method though if you suspect your bonsai tree has vine weevil larvae within it’s roots is picking out any larvae during the re-potting of your bonsai trees to avoid the need for chemical application.
A biological control can also be used in the form of the microscopic pathogenic nematodes like Steinernema kraussei or Heterorhabditis megidis which are available from suppliers of biological controls.
Also consider your bonsai mix as the larvae tend to prefer moist compost rich soils as opposed to more free draining gritty mixes.

Adult beetle stage 
Adult beetles eat the leaves of trees and plants and take out irregular notches from the leaf margins.
They affect most species and are common in mid spring up to autumn. They are around 9mm in length and have pear shaped bodies with antennae that are bent at an angle about half way along their length.
They are slow moving beetles that tend to emerge at night and do not fly. The female beetle can produce hundreds of eggs between April to September and these eggs are small around 1mm in diameter and brown.
Control of the beetle stage
Preventative applications can be sprayed onto plants leaves in the form of insecticides which contain bifenthrin to make the leaves less palatable for the adult beetle.
Adult vine weevils can be found on leaves by torchlight at night and picked off and disposed off.
Systemic application of insecticides specific for vine weevil can be effective on the adult beetle but again repeat applications may be required.
Natural predators include birds, frogs, hedgehogs and predatory ground beetles.